WASHINGTON — An experimental Ebola drug has been used to treat two American aid workers and a Spanish missionary priest. Could Liberian doctors be next?

The Liberian government said Monday that it will receive doses of the drug to treat two doctors in the country. They would be the first Africans to receive it.

The manufacturer, Mapp Biopharmaceutical Inc., said in a statement posted Monday afternoon on its website that the supply of the drug is now exhausted.

The announcement came as the World Health Organization considered ethical questions about who should get access to an experimental drug in an emergency.

Some questions and answers about the Ebola drug:

Q: What is this drug?

A: Called ZMapp, it is a cocktail of specially engineered antibodies designed to target and inactivate the Ebola virus.

Q: What do we know about whether it works?

A: Very little. Various antibodies have been tested in small numbers of monkeys, but not people. In one study, 43 per cent of treated monkeys survived when the drug was given after the animals showed symptoms.

Mapp Biopharmaceutical now is developing a combination of three antibodies that seemed most promising in those animal studies.

Q: Why isn’t ZMapp being tested more widely to find out if it works in people?

A: There’s not enough available. The antibodies are grown inside tobacco plants, and then extracted and purified, a slow process. U.S. officials have estimated that only a modest amount could be produced in two or three months, unless some way to speed production is found.

Q: What does it mean that the two American aid workers who received the drug are reported to be slowly improving?

A: Top U.S. health officials stress that there’s no way to know if the drug really helped, or if those two patients would have been among the 40 per cent of people who are surviving this outbreak anyway. Without human studies, there also isn’t any way to know if the drug might harm instead of help.

There is no proven treatment for Ebola. But basic supportive care — things like keeping patients hydrated, maintaining their blood pressure and treating any complicating infections — can make a difference in survival, says Dr. Tom Frieden, director of the Centers for Disease Control and Prevention.

In this photo taken Sunday, Aug. 10, an ambulance leaves the Ebola isolation unit carrying the bodies of Ebola victims that are highly contagious to a burial site, at the Kenema Government Hospital situated in the Eastern Province around 300 km from the capital city of Freetown in Kenema, Sierra Leone.

Q: How were the Americans and Spanish priest chosen to get some of those limited doses, rather than Africans?

A: The international relief organization Samaritan’s Purse and Emory University Hospital requested that the manufacturer provide some of the drug for the two Americans, and the manufacturer agreed. As for the Spanish missionary priest, it wasn’t clear exactly how Spanish officials obtained a dose that apparently was in Geneva. The priest also was infected in Liberia and is in isolation at a Madrid hospital.

Typically, the Food and Drug Administration doesn’t regulate shipments of unapproved drugs for emergency use in individual patients outside the U.S.

The World Health Organization is debating if any further limited supplies of experimental drugs should be used during the outbreak, and under what conditions. But the agency cannot force a manufacturer to go along. Indeed, using an experimental drug outside of a research study isn’t just a gamble for patient safety. What if a drug might benefit patients early in the disease, but doctors can’t tell because it was given only to the most gravely ill?

Q: How is Liberia getting the drug?

A: The U.S. Department of Health and Human Services said it had helped connect Liberia with the manufacturer. “Since the drug was shipped for use outside the US, appropriate export procedures had to be followed,” the statement said.

Q: Are any other drugs in the pipeline?

A: Canada’s Tekmira Pharmaceuticals Corp. is developing a drug that targets Ebola’s genetic material. The FDA had halted a small safety study with questions about a reaction in healthy volunteers. Last week, Tekmira announced that FDA had modified its restriction, clearing a roadblock to possible experimental use in infected patients; the company said at the time that it was “carefully evaluating options.”

A handful of other companies are in earlier stages of drug development; a possible vaccine to prevent the disease is expected to begin first-stage safety studies sometime in the fall.

Q: If experimental drugs won’t stop the outbreak, what will?

A: Frieden and other experts say old-fashioned public health measures: rapidly finding and isolating the sick, quarantining those exposed and educating the public on how to avoid the risk of infection. Ebola is spread through direct contact with bodily fluids of sick patients. Frieden said the two main drivers of the outbreak are improper infection control during patient care and traditional but risky burial practices that have mourners handling bodies that are still infectious.